The invention is directed to a process for the recovery of noble metals from dilute aqueous or non-aqueous solutions which contain salts of non-noble (base) metals and/or other difficultly volatile inorganic or organic compounds. Included in the recoverable noble metals are silver, gold, and the platinum group of metals, e.g. platinum, palladium, rhodium, ruthenium, and iridium.
In the chemical technology of the noble metals there accumulate in many areas aqueous and non-aqueous solutions from which the noble metal portion must be recovered with the maximum possible separation from additional ballast materials such as non-noble metal salts, neutral salts, or difficultly volatile organic compounds.
In many cases, such as in several hydrometallurgical processes for the recovery of noble metal from ores, their subsequent products or from re-cycling materials, the working up of such solutions is in the middle of the process of recovery of the noble metal. Furthermore, the wet chemical separation of noble metals (Pt, Pd, Rh, Ir, Ru, Os, Au, Ag) from each other and their separation from non-noble metals as well as the purification of noble metals leads to relatively greatly diluted waste solutions, such as mother liquors from precipitations and crystallizations or wash solutions. Their noble metal portion must be recovered because of its high value. Finally, numerous chemical processes which are carried out with the help of noble metals, e.g. in the form of catalysts, furnish noble metal waste solutions of various compositions. The profitability of such process almost always is dependent upon the ability to substantially recycle the noble metal employed.
Only in exceptional cases, above all if the solutions to be worked up are composed of only noble metal compounds and a solvent which does not boil too high does a simple or vacuum distillation lead to the isolation and sufficient concentration of the noble metal. In the presence of additional impurities, such as non-noble metal salts, neutral salts or orgnic, high boiling compounds, the introduction of the noble metal containing waste solutions into the smelting process of a noble metal melt can be a useful way of working up. All noble metals are taken up completely by the volatile lead melt, while all other components or their subsequent products in the waste gas pass over into the slag or into the sulfidic phase. The dissolved noble metals can be precipitated from aqueous solutions or solutions miscible with water by reduction to the elemental state and be supplied to know subsequent treatment procedures. This reduction can be carried out, and therewith the noble metal concentrates obtained, by means of electric current, by means of non-noble metals such as zinc, iron or aluminum, or by means of reducing compounds such as hydrozine or sodium boranate. These methods of reduction, however, have many defects, such as the frequently incomplete precipitation, the introduction of additional, waste water loading, metals and the co-reduction of copper which is present, usually in a considerable amount, the co-precipitation of hydroxides of non-noble metals, as well as the formation of inflammable hydrogen gas. Besides the reduction reactions as a rule cannot be carried out in organic solutions. For organic solutions there have been proposed special combustion and pyrolysis processes for liquid wastes from homogeneous catalytic processes of the oxo-process for conversion of the noble metals in the concentrate, which noble metals in part are present in very dilute form.
This process have the disadvantage that air poluution readily occurs and the phosphorus contained in most of these solutions remains in the ash and can lead to difficulties in working up the concentrate.
A process has become known from German AS No. 2,911,193 which is special for the recovery of rhodium from residues of the oxo process in which process the residue is reacted with sulfur or a compound splitting out sulfur. This process has the disadvantage that it cannot be used in aqueous solutions, the sulfur, moreover for the most part forms reaction products disturbing the working up and frequently they are taken up in such large amounts of organic solution that the further working up is hindered.
Therefore, it was the problem of the present invention to develop a process for the recovery of noble metals from dilute aqueous and non-aqueous solutions which contain salts of non-noble metals and/or other difficultly volatile inorganic or organic compounds in which the process is readily carried out, generally usable and should operate with high yields of noble metal, without leading to difficulty in working up of the concentrate and the other reaction products.